1. Field of the Invention
This invention relates to compositions and methods for inhibiting barite scale formation, and more particularly to compositions and methods for inhibiting barite scale formation in acidic environments.
2. Prior Art
A common problem encountered in industrial activities and activities involving transport of water or transport of aqueous mixtures is the formation of scale on equipment, particularly on the inside surfaces of conduits. Oil field brines, sea water and most commercial water contain a variety of alkaline earth metal cations, such as calcium, barium and strontium, as well as a variety anions such as bicarbonate, carbonate, sulfate, phosphate and silicate. When such ions are present in sufficient concentrations, they tend to combine to form precipitates. Scale, formed by the deposition of any of several types of such precipitates, including calcium carbonate, calcium sulfate and barium sulfate (barite), therefore tends to coat surfaces in contact with water. Buildup of such scale on, for example, the inside surfaces of conduits not only obstructs fluid flow, but also interferes with heat transfer across the surfaces, facilitates corrosion of the surfaces and harbors the growth of bacteria.
Conventionally, scale formation is inhibited by introducing a scale inhibiting composition to the aqueous environment. Various scale inhibiting compositions have been employed to inhibit scale formation, and several such compositions are illustrated in U.S. Pat. No. 4,187,245 to Redmore et al., U.S. Pat. No. 4,234,511 to Backman, and U.S. Pat. No. 4,051,110 to Quinlan. However, it has been discovered that although scale inhibiting compositions, such as those disclosed in the above-noted patents, have been found useful for inhibiting scale formation in pH neutral aqueous environments, they generally are substantially less effective in acidic environments. Thus, commercial scale inhibitors, such as the pentamethylene phosphonic acid of diethylenetriamine, that are effective in inhibiting barite scale formation in neutral pH environments at room temperature, have been found to be essentially inoperative in acidic environments. Moreover, many commercial scale inhibitors also have been found to degrade and to lose their effectiveness at elevated temperatures, e.g., temperatures greater than about 90.degree. C. or higher. In addition, elevated temperatures have been found to encourage formation of certain scales, such as calcium carbonate and calcium sulfate. Other conventional scale inhibitors, particularly polyacrylic acids such as LMW 45N, a polyacrylic acid sold by Rohm and Haas, have been employed in attempts to inhibit scale formation in acidic environments. However, even these compositions are unsatisfactory in preventing barite scale formation in acidic environments, and are even less effective in preventing formation of other scales, such as calcium sulfate, in such environments.
Increased North Sea sea water flooding activity has increased the need for developing a way to inhibit formation of barite scale in acidic environments, particularly at elevated temperatures. Since barite tenaciously adheres to surfaces and resists removal by commercial scale removers or inhibitors, it has been found to be one of the most troublesome of oil field scales. This problem has been exacerbated since the introduction of carbon dioxide to oil wells to enhance oil production tends to lower the pH of already acidic oil field brines.
Further, geothermal forces acting on oil in underground reserves heat the oil found in the North Sea oil fields so that the oil and water mixture in conduits leading from the reserves is of elevated temperature, often exceeding 140.degree. C. Accordingly, methods are needed for inhibiting barite scale formation on surfaces in high temperature, acidic, aqueous environments, such as encountered in North Sea oil fields.